The present invention is in the field of ceramic processing and relates particularly to a method for making pure glass or ceramic products by vapor deposition and subsequent sintering.
The favored commercial practice in the manufacture of very pure glass products such as glass optical waveguides is by vapor deposition. The process conventionally involves the transport of vaporized reactants to an oxidation site, e.g., a burner or hot plasma zone adjacent to a deposition substrate or within a glass deposition tube, oxidation of the reactants at the oxidation site to form a particulate oxide or soot oxidation product on the mandrel or tube, and finally processing of the deposited soot to convert it to clear glass.
The vaporization characteristics of the reactants are critical for such processing. In commercial practice, these reactants have consisted of the volatile halides or hydrides of the selected metals or metalloids, e.g., the halides or oxyhalides of silicon, phosphorus, germanium and boron. These compounds have high vapor pressures at temperatures which are easy to maintain in a vapor delivery system and are converted to pure oxides at the oxidation site. In some cases, systems which operate at temperatures above the boiling temperature of the volatile halide or oxyhalide compound at atmospheric pressure have been used.
Suggestions have been made that other volatile compounds of these metals, such as the organometallic compounds, could be used, but there has been no commercial application of this proposal in the waveguide field. GB No. 2,071,644 suggests that silanes, chloro- and alkyl-substituted silanes, and/or silicate esters can be used in vapor delivery systems for optical waveguide production, but in general the instability, high reactivity, and/or limited vapor pressures of such compounds have mitigated against their use.
It has been recognized that it would be desirable to use many of the common glass modifying oxides such as MgO, Na.sub.2 O, Al.sub.2 O.sub.3, CaO and the like in the fabrication of glass optical waveguides by vapor deposition techniques, but no satisfactory technique for incorporating these oxides in vapor deposited glass has yet been developed. Most of the main group and rare earth metals useful as modifiers in transparent glasses do not form volatile chlorides or other volatile but stable inorganic compounds. Thus, although they have potential utility as modifying dopants in glass waveguides, no successful technique for incorporating these oxides in vapor-deposited glass in high purity and with close control over concentration has been developed.
Proposals have been made to use metal alkyls as metal sources for the vapor deposition of oxides but these compounds are generally so unstable as to be hazardous. For example, the compounds trimethylaluminum, Al(CH.sub.3).sub.3 and dimethylzinc, Zn(CH.sub.3).sub.2 are volatile and reactive, but are pyrophoric and thus very dangerous to store and to use.
U.S. Pat. No. 3,801,294 discloses an early approach to the incorporation of main group modifiers in vapor deposited oxide glasses wherein direct vaporization of metal halides from the solid state is proposed. This approach is disadvantageous because high delivery system temperatures must be maintained to achieve even moderate vapor pressures, and it is difficult to control the concentrations of dopants present in the carrier gas stream.
U.S. Pat. No. 3,883,336 proposes an alternative approach for incorporating main group oxides in vapor deposited glass wherein a solution containing a soluble salt of the desired metal is nebulized and the dispersed solution directed into an oxidation flame wherein metal oxide soot is generated. Unlike solvent-free vapor phase oxidation, this approach does not reproducibly provide an oxide soot with a particle size distribution such that it can be sintered into a void-free homogeneous mixed-oxide glass or ceramic. Moreover, the solvents used are potential sources of contamination in the product. Neither particulate oxide inclusions nor contaminating impurities can be tolerated in optical waveguide glass.
U.S. Pat. Nos. 4,141,710 and 4,173,459 suggest an alternative technique for using solutions wherein a solvent carrying a thermally decomposable organic or inorganic compound of the desired metal is supplied to the inner surface of a bait tube or crucible. This process is one of thermal decomposition, rather than vapor phase oxidation, and is therefore not well suited to the fabrication of glass optical waveguides. The deposits produced by hot-surface thermal decomposition again differ significantly in morphology from the soots produced by vapor phase oxidation, and often exhibit cracking and flaking if thick. Consequently, such deposits can be very difficult to convert to defect-free glass. Thus, while the thermal decomposition of inorganic and organometallic compounds, including metal chelates such as the acetylacetonates, has been used to produce thin oxide films on glass sheet (G.B. No. 1,454,378 and G.B. No. 2,033,374A), and for metal plating (U.S. Pat. No. 3,356,527 and U.S. Pat. No. 3,049,797), such practices are not commercially adaptable to the fabrication of bulk glass products such as optical waveguides.
It is therefore a principal object of the present invention to provide a method for making glass or other pure ceramic products which permits the incorporation of main group and rare earth metal oxides into such products without undesirable voids or compositional discontinuities such as particulate oxide inclusions.
It is a further object of the invention to provide a method for fabricating glass optical waveguides incorporating such metal oxides as dopants wherein good control over the concentration of dopants in the vapor-deposited glass and high purity in the vapor deposition product can be achieved.
Other objects and advantages of the invention will become apparent from the following description thereof.